This invention relates to techniques and systems for the storage of heat and cold in aquifers, and more particularly, to systems using stored heat and cold from aquifers in heat exchange with one or more heat transfer circuits.
The need for better ways of storing energy has been recognized in recent Federal legislation and government financed programs. This includes the need for better ways of storing thermal energy, and specifically, for storing solar energy, and heat and cold from other ambient sources, including waste heat from thermal and electrical generating facilities. For example in the case of solar energy, all existing solar heating and air conditioning systems for buildings require auxiliary heat sources to carry them over long periods of cloudy or extremely cold weather.
A well known expedient is the use of both large and small bodies of water for the storage of heat and cold. Thermal stratification of water is a common phenomenon with many practical uses. For example, hot water is lighter than cold water and rises to the top of tanks, lakes and even the oceans. This phenomenon prevents mixing of hot and cold water in domestic hot water heaters, and causes circulation in gravity hot water space heating systems. However it has been considered impractical to store hot water in uninsulated underground systems, as evidenced by absence of any mention of promising possibilities for so storing hot water in any of three independent studies on solar heating made for the National Science Foundation in 1973, by Westinghouse Electric Corporation, General Electric Company, and TRW Systems Group; which are respectively disclosed in the cited reports of NSF/RANN, Solar Energy grants: Numbers C-854, C-855, and C-853. None of these reports mentions the possibility of storing a free-floating layer of hot water at the upper interface of an aquifer, nor in confined aquifers.
The method of using aquifers for the storage of water underground which is subsequently retrieved and circulated for the heating of residences, buildings or building complexes, and other related entities, or for various process applications, is taught in U.S. Pat. No. 3,931,851 which issued to William B. Harris, and Richard R. Davison on Jan. 20, 1976 for Liquid Aquifer Energy Storage Method. Other U.S. patents relating to this field of art are Nos. 3,620,206 (Harris, et al 1971); 3,339,629 (Hervey, 1967); 2,693,939 (Marchant, et al, 1954); 2,780,415 (Gay, 1957); 2,584,573 (Gay, 1952); 2,007,406 (Miller, 1935); 2,637,531 (Davidson, 1953); 3,262,493 (Hervey, 1966); and the references cited in each of them. Also of interest is an article by Charles F. Meyer and David K. Todd entitled "Conserving Energy With Heat Storage Wells", Volume 7 Environmental Science and Technology 512 (1973).
Problems arise with the systems disclosed by Harris and others of the prior art in that they do not take into account changes in the hydraulic gradient which may occur in the area surrounding the wells; or the fact that, particularly in areas near the coast, the aquifers may be filled with salt water except for a lens of fresh water floating on the salt water. Further, on the coast and in other areas, the aquifers may be included as part of certain geological formations, such as, for example, strata sloping seaward, in such a favorable manner as to isolate the cold fresh water in a particular position with relation to the slope of the aquifer and the interface of salt and fresh water. Further, it will be apparent that there is a certain amount of seepage from ground level into the area of the storage wells, and from the area of the hot or cold water storage wells into the surroundings, or vice versa; and it is apparent that significant stored thermal energy will be lost to the atmosphere by conduction through thin layers of soil from shallow aquifers. None of these factors have been taken into account in the disclosures of the prior art, nor is there any teaching in the prior art of any techniques or systems for controlling the thermal or hydraulic gradients in the area of the wells to provide systems, the operation of which are sufficiently predictable to be of practical value in all cases.
Further problems may arise in aquifer systems because of the possibility of fouling or contamination of the natural ground water in the area by importing into the aquifer storage areas water from conduits which are open to above-ground contamination. Also, difficulties have arisen in prior art systems in controlling the energy efficiency of above-ground subsystems for collecting and/or dissipating heat and cold derived from or to be stored in the aquifers.
In this application, and in the prior art, the term `aquifer` is defined as a water-bearing bed or stratum of permeable rock, sand or gravel capable of yielding considerable quantities of water to wells or springs.